peg::parser! { grammar memo() for str {
    #[cache]
    rule r() -> &'input str
        = s:$(['a'..='z']+) { s }

    pub rule parse()
        = r() "+" r() {}
        / r() " " r() {}
}}

#[test]
fn main() {
    assert_eq!(memo::parse("abc zzz"), Ok(()));
}
